In machine tool operations it is economically advantageous to recover cutting oil which is mixed with metal chips, turnings and large chunks of metal and the like generated by these operations. Means for separating the metal chips from oils are well known in the machine tool operations industry. Centrifuge separators are typically used. See, for example, U.S. Pat. Nos. 4,310,417, 4,137,176 and 3,850,814.
A well recognized problem with centrifuge separators is that they are easily and quickly damaged by the large chunks of metal, for instance, scrap parts and bar ends, which often are included in a stream of metal chips from such operations. Large pieces of metal will severely damage centrifuge rotors. In the past, this possibility required either that the mixture of metal chips, turnings, chunks and cutting oils be visually inspected before being fed into the centrifuge, or that frequent shutdowns and repairs to the centrifuge be tolerated. Either alternative increases the cost of operating such a system.
Several systems have been devised to separate the damaging large chunks of metal from the stream of oily metal chips before the stream is introduced into the centrifuge. These systems include the use of a means of air classification of the material in a metal chip stream prior to crushing. These earlier systems are very complicated and expensive which has discouraged their use.
U.S. Pat. No. 4,310,417 discloses a chute for feeding chips into a centrifuge or other separating equipment where an opening is provided on the bottom of said chute through which large pieces of metal can drop before such pieces are introduced into the centrifuge. In the chute of U.S. Pat. No. 4,310,417, small chips are prevented from falling through the opening by a strong current of air drawn through the opening by an appropriately equipped centrifuge or by another blower-like device attached to the downstream end of the chute. A pivotal gate in the chute upstream of the opening provides a partial restriction against air being drawn into the chute upstream of the opening while periodically permitting metal chips and metal chunks to be introduced into the chute separator.
While the invention of U.S. Pat. No. 4,310,417 was a significant improvement over prior separator systems, it was not entirely reliable as a chunk separator and was subject to surging or pulsing due to non-uniform feed rates to the classifier.
When the movement of the chips is by gravity, the rate of flow can become irregular resulting in a pulse type of flow with chips moving through the chute in large masses. Under such circumstances some large pieces of metal can "wash over" the opening and be fed into the centrifuge. In addition, when large amounts of metal chips are fed through the pivotal gate, the gate is opened sufficiently to allow significant quantities of air into the conduit, causing the air velocity through the opening in the bottom of the chute to decrease. This results in small metal chips falling through said opening.
Even when the device of U.S. Pat. No. 4,310,417 is fed by appropriate means to prevent the wash over described above, the separation is only partial and not efficient. A significant fraction of the small chips falls through the opening because of problems created when the pivotal gate opens i.e. the air velocity of the air entering through the chunk discharge opening decreases temporarily, creating a pulsing effect.